In thermal conductive sheets that release heat of electronic components, both high thermal conductivity and insulating properties are required. Boron nitride is a highly thermal conductive substance having insulating properties. One form of boron nitride is a scaly particle. In a scaly boron nitride particle, thermal conductivity in a direction parallel to a scaly surface (that is, a surface direction of a thermal conductive sheets) tends to be high, but thermal conductivity in a direction orthogonal to a scaly surface (that is, a thickness direction of a thermal conductive sheets) tends to be low.
Thus, in order to improve thermal conductivity of a thermal conductive sheets in a film thickness direction, it is preferable that boron nitride “stand” in the sheet.
As a technique for causing boron nitride to “stand,” there is a technique using granules obtained by granulating boron nitride primary particles. However, when a pressure is applied to thermal conductive sheets interposed between a heat source and a heat dissipation member, granules may collapse, boron nitride primary particles may fall down, and the thermal conductivity may be lowered.
Patent Literature 1 discloses a technique in which hard boron nitride granules having low porosity are used so that the collapse of boron nitride granules due to a pressure is prevented and a reduction in thermal conductivity is limited.
Patent Literature 2 discloses a technique in which soft and easily deformable boron nitride granules are used, deformation is performed to an extent that the granules do not completely collapse, a pressure is thus relieved, and a reduction in thermal conductivity is limited.
Patent Literature 3 discloses a technique in which a smooth adhesive layer is bonded to a layer containing a thermal conductive filler such as boron nitride to fill irregularities on the surface of the layer containing the thermal conductive filler, and thus the adhesive strength is improved.
Patent Literature 4 discloses a multi-layer resin sheet including a resin layer that contains 3 types of thermal conductive fillers with different sizes, and an adhesive layer that is disposed on at least one surface of the resin layer. In addition, this literature discloses that the adhesive layer may also contain a filler such as aluminum oxide.